Assay methods of the transformation of primary rat embryo cells by carcinogenic fluorenylhydroxamic acids are being developed. Sparse monolayers or suspension cultures maintained in an atmosphere containing 5% CO2 were exposed once to N-acetoxy-2-fluorenylacetamide. The concentrations of carcinogen ranged from 2.5 to 10.0 nmoles/10 to the 6th power cells. Treatment of the sparse monolayers gave high cloning efficiencies but low transformation frequencies. Suspension cultures treated in the same way exhibited low cloning efficiencies but relatively high transformation frequencies. Experiments are in progress to define optimal conditions for the assay system.